Magnification

In the experiments with lenses you have done you should have noticed a couple of things about the image.

1. It is inverted and

2. It may be bigger or smaller than the object. The magnification M = image size / object size

Notice how light from the top of the object creates the bottom of the image and vice versa.

From geometry you should also see that:

Consider this example:

A lens of focal length 5cm produces an image of a 30cm tall cat 80cm away.
Calculate a) the power of the lens, b) the distance of the image from the lens and c) the height of the cat in the image.

P = 1 / f   = 1 / 0.05  = 20D

f = 5cm and u = -80cm. Using the lens equation 1/v = -1/80 + 1/5  which gives v = 5.33cm

M = v / u = 5.33 / 80 = 66.6 x 10^-3  so    I = 30 x M     = 1.99cm